Songwriting question

Say I want to write a song in a basic C Am Dm G progression. Should I develop a strumming pattern first and then put the vocal melody line on top of it or I should come up with a vocal line first and then try to develop a rhythm that fits the vocal line?

I always write my lyric's first because its easier for me to figure out a progression and rhythm to match the words, but everybody does it different like joey said there really isn't any right or wrong way. I'd read all the stickies here too if you haven't done that yet

You go back and forth altering the two. Start with whichever; you're going to have to continuously change your vocal melody until it is smooth enough, you're going to have to continuously change your strumming pattern to develop a rhythm pattern that works with the lyrics and melody. I doesn't really matter which one you write first when you'e going to re-write both to suit each other a half dozen times.